Gas turbine engines of the so-called ducted-fan type are well known and widely used for powering aircraft, and commercial aircraft in particular. Engines of this type have a relatively large propulsive fan comprising a plurality of generally radial fan blades and which is mounted for rotation about the principle rotational axis of the engine within a nacelle comprising a fan case. The principle functions of the fan case include; defining the outer annulus of the gas flow through the engine and containment of a fan blade should it disintegrate or become detached from the fan during flight. The fan case thus includes a fan track liner which circumscribes the fan, and which is configured to absorb the energy of a detached fan blade to prevent the blade passing through the core of the engine. The radially innermost surface of the fan track liner is typically provided in very close proximity to the outer tips of the fan blades in order to maintain an effective seal across the fan, for reasons of engine efficiency and performance. This is usually achieved by configuring the fan track liner so that it is abradable by the tips of the blades to maintain tight blade tip clearances.
As will be appreciated, given the importance of the requirement for the fan track liner to absorb the energy of a detached fan blade, the integrity of the liner is very important. It is thus considered important to check and monitor the integrity of the fan track liner during the service life of an engine. Current methods and techniques for inspecting the integrity of the fan track liner during service are considered to be less than optimal in terms of efficiency and convenience. For example, it is common for inspection of an engine's fan track liner to require complete removal of the fan from the engine in order to provide sufficient access to the fan track liner.